Electromagnetic radiation from the sun continuously bombards the Earth's atmosphere. Light is made up of electromagnetic radiation that travels in waves. The electromagnetic spectrum includes radio waves, millimeter waves, microwaves, infrared, visible light, ultra-violet (UVA and UVB), x-rays, and gamma rays. The visible light spectrum includes the longest visible light wavelength of approximately 700 nm and the shortest of approximately 400 nm (nanometers or 10−9 meters). Blue light wavelengths fall in the approximate range of 400 nm to 500 nm. For the ultra-violet bands, UVB wavelengths are from 290 nm to 320 nm, and UVA wavelengths are from 320 nm to 400 nm. Gamma and x-rays make up the higher frequencies of this spectrum and are absorbed by the atmosphere. The wavelength spectrum of ultraviolet radiation (UVR) is 100 nm to 400 nm. Most UVR wavelengths are absorbed by the atmosphere, except where there are areas of stratospheric ozone depletion. Over the last 20 years, there has been documented depletion of the ozone layer primarily due to industrial pollution. Increased exposure to UVR has broad public health implications as an increased burden of UVR ocular and skin disease is to be expected.
The ozone layer absorbs wavelengths up to 286 nm, thus shielding living beings from exposure to radiation with the highest energy. However, we are exposed to wavelengths above 286 nm, most of which falls within the human visual spectrum (400-700 nm). The human retina responds only to the visible light portion of the electromagnetic spectrum. The shorter wavelengths pose the greatest hazard because they inversely contain more energy. Blue light has been shown to be the portion of the visible spectrum that produces the most photochemical damage to animal retinal pigment epithelium (RPE) cells. Exposure to these wavelengths has been called the blue light hazard because these wavelengths are perceived as blue by the human eye.
Cataracts and macular degeneration are widely thought to result from photochemical damage to the intraocular lens and retina, respectively. Blue light exposure has also been shown to accelerate proliferation of uveal melanoma cells. The most energetic photons in the visible spectrum have wavelengths between 380 and 500 nm and are perceived as violet or blue. The wavelength dependence of phototoxicity summed over all mechanisms is often represented as an action spectrum, such as is described in Mainster and Sparrow, “How Much Blue Light Should an IOL Transmit?” Br. J. Opthalmol., 2003, v. 87, pp. 1523-29 and FIG. 6. In eyes without an intraocular lens (aphakic eyes), light with wavelengths shorter than 400 nm can cause damage. In phakic eyes, with some intraocular devices, this light is absorbed by the intraocular lens and therefore does not contribute to retinal phototoxicity; however it can cause optical degradation of the lens or cataracts.
The pupil of the eye responds to the photopic retinal illuminance, in trolands, which is the product of the incident flux with the wavelength-dependent sensitivity of the retina and the projected area of the pupil. This sensitivity is described in Wyszecki and Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae (Wiley: New York) 1982, esp. pages 102-107.
Current research strongly supports the premise that short wavelength visible light (blue light) having a wavelength of approximately 400 nm-500 nm could be a contributing cause of AMD (age related macular degeneration). It is believed that the highest level of blue light absorption occurs in a region around 430 nm, such as 400 nm-460 nm. Research further suggests that blue light worsens other causative factors in AMD, such as heredity, tobacco smoke, and excessive alcohol consumption.
High energy blue/violet light, e.g., light having wavelengths of about 400 nm to about 470 nm, either by itself or in combination with ultraviolet light, may contribute to oxidative changes within the human crystalline lens resulting in presbyopia. Presbyopia is the loss of the ability (or accommodative amplitude) of the human crystalline lens to focus at near point. This loss of accommodation begins at infancy and increases throughout life, but it goes unnoticed by the individual until around age 40-45 years when he or she experiences near-point blur when viewing a near-point object. Today, presbyopia affects an estimated 1.6 billion people worldwide, and this number is expected to increase as the population ages.
The human retina includes multiple layers. These layers listed in order from the first exposed to any light entering the eye to the deepest include:
1) Nerve Fiber Layer
2) Ganglion Cells
3) Inner Plexiform Layer
4) Bipolar and Horizontal Cells
5) Outer Plexiform Layer
6) Photoreceptors (Rods and Cones)
7) Retinal Pigment Epithelium (RPE)
8) Bruch's Membrane
9) Choroid
When light is absorbed by the eye's photoreceptor cells, (rods and cones) the cells bleach and become unreceptive until they recover. This recovery process is a metabolic process and is called the “visual cycle.” Absorption of blue light has been shown to reverse this process prematurely. This premature reversal increases the risk of oxidative damage and is believed to lead to the buildup of the pigment lipofuscin in the retina. This build up occurs in the retinal pigment epithelium (RPE) layer. It is believed that aggregates of extra-cellular materials called drusen are formed due to the excessive amounts of lipofuscin.
Current research indicates that over the course of one's life, beginning with that of an infant, metabolic waste byproducts accumulate within the pigment epithelium layer of the retina, due to light interactions with the retina. This metabolic waste product is characterized by certain fluorophores, one of the most prominent being lipofuscin constituent A2E. In vitro studies by Sparrow indicate that lipofuscin chromophore A2E found within the RPE is maximally excited by 430 nm light. It is theorized that a tipping point is reached when a combination of a build-up of this metabolic waste (specifically the lipofuscin fluorophore) has achieved a certain level of accumulation, the human body's physiological ability to metabolize within the retina certain of this waste has diminished as one reaches a certain age threshold, and a blue light stimulus of the proper wavelength causes drusen to be formed in the RPE layer. It is believed that the drusen then further interfere with the normal physiology/metabolic activity which allows for the proper nutrients to get to the photoreceptors thus contributing to age-related macular degeneration (AMD). AMD is the leading cause of irreversible severe visual acuity loss in the United States and Western World. The burden of AMD is expected to increase dramatically in the next 20 years because of the projected shift in population and the overall increase in the number of ageing individuals.
Drusen hinder or block the RPE layer from providing the proper nutrients to the photoreceptors, which leads to damage or even death of these cells. To further complicate this process, it appears that when lipofuscin absorbs blue light in high quantities it becomes toxic, causing further damage and/or death of the RPE cells. It is believed that the lipofuscin constituent A2E is at least partly responsible for the short-wavelength sensitivity of RPE cells. A2E has been shown to be maximally excited by blue light; the photochemical events resulting from such excitation can lead to cell death. See, for example, Janet R. Sparrow et al., “Blue light-absorbing intraocular lens and retinal pigment epithelium protection in vitro,” J. Cataract Refract. Surg. 2004, vol. 30, pp. 873-78.
From a theoretical perspective, the following appears to take place:
1) Waste buildup occurs within the pigment epithelial level starting from infancy through out life.
2) Retinal metabolic activity and ability to deal with this waste typically diminish with age.
3) The macula pigment typically decreases as one ages, thus filtering out less blue light.
4) Blue light causes the lipofuscin to become toxic. The resulting toxicity damages pigment epithelial cells.
The lighting and vision care industries have standards as to human vision exposure to UVA and UVB radiation Surprisingly, no such standard is in place with regard to blue light. For example, in the common fluorescent tubes available today, the glass envelope mostly blocks ultra-violet light but blue light is transmitted with little attenuation. In some cases, the envelope is designed to have enhanced transmission in the blue region of the spectrum. Such artificial sources of light hazard may also cause eye damage. The inventors theorize that the combination of the ozone hole in the atmosphere, numerous near-point tasks such as working on a computer, various artificial lighting including specifically florescent tubes and blue diodes, television sets, and even camera flashes contribute to the proliferation of damaging wavelengths of UV and high energy blue (blue/violet) light.
Laboratory evidence by Sparrow at Columbia University has shown that if about 50% of the blue light within the wavelength range of 430±30 nm is blocked, RPE cell death caused by the blue light may be reduced by up to 80%. External eyewear such as sunglasses, spectacles, goggles, and contact lenses that block blue light in an attempt to improve eye health are disclosed, for example, in U.S. Pat. No. 6,955,430 to Pratt. Other ophthalmic devices whose object is to protect the retina from this phototoxic light include intraocular and contact lenses. These ophthalmic devices are positioned in the optical path between environmental light and the retina and generally contain or are coated with dyes that selectively absorb blue and violet light.
Other lenses are known that attempt to decrease chromatic aberration by blocking blue light. Chromatic aberration is caused by optical dispersion of ocular media including the cornea, intraocular lens, aqueous humour, and vitreous humour. This dispersion focuses blue light at a different image plane than light at longer wavelengths, leading to defocus of the full color image. Conventional blue blocking lenses are described in U.S. Pat. No. 6,158,862 to Patel et al., U.S. Pat. No. 5,662,707 to Jinkerson, U.S. Pat. No. 5,400,175 to Johansen, and U.S. Pat. No. 4,878,748 to Johansen.
Conventional methods for reducing blue light exposure of ocular media typically completely occlude light below a threshold wavelength, while also reducing light exposure at longer wavelengths. For example, the lenses described in U.S. Pat. No. 6,955,430 to Pratt transmit less than 40% of the incident light at wavelengths as long as 650 nm, as shown in FIG. 6 of Pratt '430. The blue-light blocking lens disclosed by Johansen and Diffendaffer in U.S. Pat. No. 5,400,175 similarly attenuates light by more than 60% throughout the visible spectrum, as illustrated in FIG. 3 of the '175 patent.
Balancing the range and amount of blocked blue light may be difficult, as blocking and/or inhibiting blue light affects color balance, color vision if one looks through the optical device, and the color in which the optical device is perceived. For example, shooting glasses appear bright yellow and block blue light. The shooting glasses often cause certain colors to become more apparent when one is looking into a blue sky, allowing for the shooter to see the object being targeted sooner and more accurately. While this works well for shooting glasses, it would be unacceptable for many ophthalmic applications. In particular, such ophthalmic systems may be cosmetically unappealing because of a yellow or amber tint that is produced in lenses by blue blocking. More specifically, one common technique for blue blocking involves tinting or dyeing lenses with a blue blocking tint, such as BPI Filter Vision 450 or BPI Diamond Dye 500. The tinting may be accomplished, for example, by immersing the lens in a heated tint pot containing a blue blocking dye solution for some predetermined period of time. Typically, the dye solution has a yellow or amber color and thus imparts a yellow or amber tint to the lens. To many people, the appearance of this yellow or amber tint may be undesirable cosmetically. Moreover, the tint may interfere with the normal color perception of a lens user, making it difficult, for example, to correctly perceive the color of a traffic light or sign.
Efforts have been made to compensate for the yellowing effect of conventional blue blocking filters. For example, blue blocking lenses have been treated with additional dyes, such as blue, red or green dyes, to offset the yellowing effect. The treatment causes the additional dyes to become intermixed with the original blue blocking dyes. However, while this technique may reduce yellow in a blue blocked lens, intermixing of the dyes may reduce the effectiveness of the blue blocking by allowing more of the blue light spectrum through. Moreover, these conventional techniques undesirably reduce the overall transmission of light wavelengths other than blue light wavelengths. This unwanted reduction may in turn result in reduced visual acuity for a lens user.
It has been found that conventional blue-blocking reduces visible transmission, which in turn stimulates dilation of the pupil. Dilation of the pupil increases the flux of light to the internal eye structures including the intraocular lens and retina. Since the radiant flux to these structures increases as the square of the pupil diameter, a lens that blocks half of the blue light but, with reduced visible transmission, relaxes the pupil from 2 mm to 3 mm diameter, will actually increase the dose of blue photons to the retina by 12.5%. Protection of the retina from phototoxic light depends on the amount of this light that impinges on the retina, which depends on the transmission properties of the ocular media and also on the dynamic aperture of the pupil. Previous work to date has been silent on the contribution of the pupil to prophylaxis of phototoxic blue light.
Another problem with conventional blue-blocking is that it can degrade night vision. Blue light is more important for low-light level or scotopic vision than for bright light or photopic vision, a result which is expressed quantitatively in the luminous sensitivity spectra for scotopic and photopic vision. Photochemical and oxidative reactions cause the absorption of 400 to 450 nm light by intraocular lens tissue to increase naturally with age. Although the number of rod photoreceptors on the retina that are responsible for low-light vision also decreases with age, the increased absorption by the intraocular lens is important to degrading night vision. For example, scotopic visual sensitivity is reduced by 33% in a 53 year-old intraocular lens and 75% in a 75 year-old lens. The tension between retinal protection and scotopic sensitivity is further described in Mainster and Sparrow, “How Much Light Should and IOL Transmit?” Br. J. Opthalmol., 2003, v. 87, pp. 1523-29.
Conventional approaches to blue blocking also may include cutoff or high-pass filters to reduce the transmission below a specified blue or violet wavelength to zero. For example, all light below a threshold wavelength may be blocked completely or almost completely. For example, U.S. Pub. Patent Application No. 2005/0243272 to Mainster and Mainster, “Intraocular Lenses Should Block UV Radiation and Violet but not Blue Light,” Arch. Ophthal., v. 123, p. 550 (2005) describe the blocking of all light below a threshold wavelength between 400 and 450 nm. Such blocking may be undesirable, since as the edge of the long-pass filter is shifted to longer wavelengths, dilation of the pupil acts to increase the total flux. As previously described, this can degrade scotopic sensitivity and increase color distortion.
Recently there has been debate in the field of intraocular lenses (IOLs) regarding appropriate UV and blue light blocking while maintaining acceptable photopic vision, scotopic vision, color vision, and circadian rhythms.
In view of the foregoing, there is a need for an ophthalmic system that can provide one or more of the following:
1) Blue blocking with an acceptable level of blue light protection
2) Acceptable color cosmetics, i.e., it is perceived as mostly color neutral by someone observing the ophthalmic system when worn by a wearer.
3) Acceptable color perception for a user. In particular, there is a need for an ophthalmic system that will not impair the wearer's color vision and further that reflections from the back surface of the system into the eye of the wearer be at a level of not being objectionable to the wearer.
4) Acceptable level of light transmission for wavelengths other than blue light wavelengths. In particular, there is a need for an ophthalmic system that allows for selective blockage of wavelengths of blue light while at the same time transmitting in excess of 80% of visible light.
5) Acceptable photopic vision, scotopic vision, color vision, and/or circadian rhythms.
6) Improved contrast sensitivity and/or visual acuity.
This need exists as more and more data is pointing to blue light as one of the possible contributory factors in macula degeneration (the leading cause of blindness in the industrialized world) and also other retinal diseases, including cataracts (the leading cause of blindness in the non-industrialized world) and presbyopia.